The thickening cloud of volcanic ash over Europe caused some world leaders - including President Obama, German Chancellor Angela Merkel, French President Nicolas Sarkozy and Canadian Prime Minister Stephen Harper - to cancel plans to attend today's state funeral. Still, some European leaders said they would drive to Krakow.

Obama "waited as long as possible before he made the decision because he wanted to come," said Lee Feinstein, U.S. Ambassador to Poland. "But it was impossible for him to travel."

The crowd in Warsaw's Pilsudski Square waved white-and-red Polish flags with black ribbons of mourning affixed to them. A massive white stage, a large cross in the center, was flanked by oversized photos of the dead, including President Lech Kaczynski.

The names of the dead were read aloud, starting with the president and his wife, Maria, while Marta, their only child, and Jaroslaw Kaczynski, the president's twin brother and former prime minister, looked on. Others at the service included former President Lech Walesa, Prime Minister Donald Tusk and acting president Bronislaw Komorowski.

"Our world went crashing down for the second time at the same place," Komorowski said of the crash near Russia's Katyn forest, site of a World War II massacre of Polish officers.